This invention relates to lithographic printing plates and, more particularly, to novel photosensitive coatings for negative working lithographic printing plates which, upon irradiation, are developable by aqueous alkaline solutions.
The art of lithographic printing depends upon the immiscibility of grease and water, upon the preferential retention of a greasy image-forming substance by an image area, and upon the similar retention of an aqueous dampening fluid by a non-image area. When a greasy image is imprinted upon a suitable surface and the entire surface is then moistened with an aqueous solution, the image area will repel the water and the non-image area will retain the water. Upon subsequent application of greasy ink, the image portion retains ink whereas the moistened non-image area repels it. The ink on the image area is then transferred to the surface of a material on which the image is to be reproduced, such as paper, cloth and the like, via an intermediary, a so-called offset or blanket cylinder, which is necessary to prevent mirror-image printing.
The type of lithographic plate to which the present invention is directed has a coating of a light-sensitive substance that is adherent to an aluminum base sheet. If the light-sensitive coating is applied to the base sheet by the manufacturer, the plate is referred to as a "presensitized plate." If the light-sensitive substance is applied to the base by the lighographer or trade platemaker, the plate is referred to as a "wipe-on" plate. Depending upon the nature of the photosensitive coating employed, the treated plate may be utilized to reproduce directly the image to which it is exposed, in which case it is termed a positive-acting plate, or to produce an image complementary to the one to which it is exposed, in which case it is termed a negative-acting plate. In either case the image area of the developed plate is oleophilic and the non-image area is hydrophilic.
In the case of a negative plate that is exposed to light through a negative transparency, the light-sensitive material, commonly a diazo compound, is caused to harden and thereby become insoluble in a desensitizing solution applied to the plates after light exposure for the purpose of removing that part of the light-sensitive coating which, because it was protected from the light by the negative, was not light-hardened. The light-hardened surface of a negative plate will be the oleophilic surface compatible with the greasy ink and is called the "image area;" the surface from which the non-hardened light-sensitive material has been removed by a desensitizer will be, or can be converted to, a hydrophilic surface having little affinity for the greasy ink and is called the "non-image" area.
A positive plate is generally one upon which the non-image area is the portion of the light-sensitive diazo compound exposed to light while the unexposed portion is either oleophilic or adapted to be converted by chemical reaction to a hardened oleophilic ink-receptive image area.
It is known in the art to produce positive working lithographic printing plates which may be developed with aqueous alkaline developers, however, to date negative type presensitized plates required organic solvent developers which are undesirable polluting agents.